1. Field of the Invention
The present invention relates to an etching solution reproducing apparatus for reproducing waste etching solution after wet etching treatment and an etching solution reproduction method.
2. Description of Related Art
The wet etching is an art for removing film formed on a substrate using chemical dissolution action of etching solution and has been used in various kinds of technical fields. Particularly in field of semiconductor, it has been used as an art for processing the shape of thin film on a semiconductor such as wafer and removes unnecessary thin film by etching after thin film such as oxide film is formed on a semiconductor wafer so as to form a pattern with photo resist. When producing a semiconductor device on a silicon wafer, phosphoric acid aqueous solution heated at 150 to 175° C. is used as the etching solution in a process for removing silicon nitride film from a silicon wafer on which the silicon nitride film (Si3N4) and silicon oxide (SiO2) are formed.
FIG. 1 is an explanatory diagram showing a conventional art of the etching apparatus described in the Japanese Patent No. 3842657. Its basic configuration includes a structure for circulating etching solution consisted of phosphoric acid aqueous solution (H3PO4+H2O) within an etching bath 1. The etching bath 1 includes two-bath (external/internal) structure constituted of an internal bath 1A having a volume capable of accommodating plural pieces of silicon wafers W vertically in parallel and provided with a heater 1H on its side portion and an overflow bath 1B formed to surround the outer periphery thereof. The internal bath 1A and the bottom portion of the overflow bath 1B are connected to each other through a circulation passage having a circulation pump 2, a filtration filter 3 and a line heater 4.
Etching solution within the etching bath 1 is controlled at 150 to 175° C. by the heater 1H and etching solution overflowing into the overflow bath 1B is sent by pressurization by the circulation pump 2 and deprived of foreign matter such as dust by the filtration filter 3. After that, the solution is reheated to the aforementioned temperature by a line heater 4 and returned to the internal bath 1A. Silicon wafers W dipped in the etching bath 1 are processed by etching solution whose temperature is controlled and from which foreign matter is removed so as to carry out etching treatment of removing silicon nitride selectively from the silicon wafer W on which the silicon nitride film and silicon oxide film are formed.
If the aforementioned etching treatment is repeated, silicon compound is generated in etching solution by the etching treatment of silicon nitride film and accumulated gradually so that concentration of silicon compound in phosphoric acid aqueous solution is increased. Consequently, silicon compound is precipitated into phosphoric acid solution so as to turn to foreign matter and causes clogging of the filtration filter 3 in the circulation passage. By change in concentration of silicon compound in phosphoric acid aqueous solution, etching selectivity expressed by a ratio of etching amount per unit time of each of silicon nitride film and silicon oxide film of each treatment lot is changed, so that etching treatment on plural processing lots with a constant selectivity is disabled.
To solve this problem, according to the conventional art shown in FIG. 1, another circulation passage branching from the aforementioned main circulation passage is formed and a removing device 5 for removing silicon compound generated in etching solution by forced precipitation is connected to this circulation passage. Consequently, this can inhibit increase of the concentration of silicon compound in etching solution circulated during etching treatment to some extent.
According to such a conventional art, although increase of silicon compound in etching solution can be suppressed to some extent, accumulation of silicon compound in etching solution step by step is unavoidable in a process of etching processing of a large amount of silicone wafer, because the conventional art cannot remove silicon compound in a high removal rate. Therefore, the conventional art cannot help replacing the etching solution with new phosphoric acid aqueous solution periodically, finally. As a result, an increase of processing cost due to utilization of the new phosphoric acid aqueous solution is problematic and further, treatment of waste etching solution which is industrial waste is also problematic.
As another conventional art, inclusion of a process of mixing hydrofluoric acid or ultra-pure water in waste etching solution in the phosphoric acid aqueous solution reproduction process has been proposed. Although removal of hydrofluoric acid is extremely important because remaining hydrofluoric acid accelerates etching of silicon oxide film, it is extremely difficult to remove hydrofluoric acid from waste etching solution and verify to what extent the hydrofluoric acid is removed. Further, although ultra-pure water is an indispensable component for etching treatment process of silicon nitride film, the quantity of ultra-pure water mixed is increased. As a result, excessive ultra-pure water needs to be removed before the etching solution is recycled, thereby producing an energy loss, which is a problem.
On the other hand, just after etching solution is replaced with new phosphoric acid aqueous solution, an etching amount of silicon oxide film per unit time is increased by the new phosphoric acid aqueous solution. As a result, sometimes, etching treatment cannot be carried out based on a desired etching selectivity. To avoid this phenomenon, sometimes, dummy etching of dipping a dummy wafer in which silicone nitride film is generated in etching solution is intentionally carried out in order to raise the concentration of silicon compound in etching solution. Consequently, processing time is increased by the dummy etching whereby disabling high productivity etching treatment.
Although to avoid the dummy etching, it has been considered to discharge part of waste etching solution while measuring and adjusting the concentration of silicon compound and replenish a specified amount of new etching solution (for example, Japanese Patent Application Laid-Open No. 2001-23952), such a fundamental problem that treatment cost is increased by adding new etching solution and waste etching solution as industrial waste needs to be disposed has not been solved and it has been desired to adjust the concentration of silicon compound of etching solution by carrying out waste etching solution treatment which does not affect natural environment badly.